This clinical study tests the hypothesis that coronary blood flow dynamics of patients may be altered in the immediate interval after a coronary bypass procedure which may be deleterious to the patient's future course if unrecognized. A thermal dilution catheter is placed into the coronary sinus percutaneously and threaded to the great cardiac vein. Ports located at the tip allow sampling of the anterior myocardium and those more proximal sample the entire left ventricle. Blood samples are used to determine regional oxygen consumption, acid base balance, lactic acid, pyruvate, creatine kinase, and lactic dehydrogenase isoenzyme concentrations. Complete hemodynamic evaluations are performed preoperatively and in serial fashion for 6-8 hours after operation. Six patients have had complete studies. In some, no significant changes in coronary vascular resistance, cardiac output, or any of the biochemical variables occur. Other patients show changes which can be related to less than optimal contractility and systemic flow. Control of heart rate through atrial pacing and preload may eliminate some of this variability. Animal studies indicate that the use of a fiberoptic, thermodilution catheter will allow on-line documentation of myocardial metabolic trends. Such continuous measurement will allow better assessment of coronary flow dynamics in future patients studies.